1. Field of the Invention
The present invention is directed to a method for increasing wear resistance of a workpiece surface which has been provided with a converted layer and to such a workpiece with improved wear resistance.
2. Description of Prior Art
For a great variety of workpieces, as e.g. for forming tools, it is advantageous to subject their surfaces to a surface treatment, thereby forming a converted layer. This may e.g. be done by a thermo-chemical treatment or by vacuum plasma treatment. This treatment will be referred to as "annealing" and is meant to include such thermo-chemical or vacuum treatments.
On the other hand, for a multitude of workpieces, a coating of their surfaces with a hard material layer is advantageous for increasing their wear resistance.
Up to now, it has only been possible to a very restricted extent to combine these two methods. In spite of the fact that here and there positive results were reported, the pre-annealed and thereafter hard material coated workpieces showed in practice binding problems with respect to the hard material coatings which resulted in that, at least locally, hard coating disruptures occurred with a respective loss of hard coating along surfaces of such combined treated workpieces during their practical use.
If there is applied on an annealed surface, annealed e.g. thermo-chemically or by vacuum treatment, as for instance by a physical vapor deposition process, a hard material, wear resistance layer, there is formed along the annealing layer a thin layer whereat the content of semi-metal of the annealing layer, which is characteristic for such an annealing layer, is lowered to show a significant lack of semi-metal content.
We understand under semi-metals especially the elements C, N, B, O, S, Si, but not exclusively.
This thin layer with lack of semi-metal content, which shall be named "intermediate layer", shows only a small tension and shearing strength. It is due to this intermediate layer that the applied hard material layer is either sheared off during practical use or peels off due to pressure tension inherently prevailing in the hard material coating. Nevertheless, it must be admitted that this drawback does not always occur. It appears that some method and material combinations exist, whereat this phenomenon of badly adhering hard material layer may not be observed, at least not as pronounced as was described. Nevertheless, the mere fact that these problems do occur at most method and material combinations is a tremendous drawback for refining workpieces, e.g. in a refining centre, where a great variety of different workpieces is to be treated.
Up to now, the attempts to resolve the said problem of badly adhering hard material coating were limited on reducing the temperature during hard material coating, e.g. below 300.degree. C. At such low temperatures it is nevertheless not possible to deposit hard material layers with a sufficient adherence by known physical vapor deposition processes (PVD).